Effects of elevated sulfate concentration on the mobility of arsenic in the sediment-water interface.

The adsorption/desorption of arsenic (As) at the sediment-water interface in lakes is the key to understanding whether As can enter the ecosystem and participate in material circulation. In this study, the concentrations of As(III), total arsenic [As(T)], sulfide, iron (Fe), and dissolved organic carbon (DOC) in overlying water were observed after the initial sulfate (SO) concentrations were increased by four gradients in the presence and absence of microbial systems. The results indicate that increased SO concentrations in overlying water triggered As desorption from sediments. Approximately 10% of the desorbed As was desorbed directly as arsenite or arsenate by competitive adsorption sites on the iron salt surface; 21% was due to the reduction of iron (hydr)oxides; and 69% was due to microbial activity, as compared with a system with no microbial activity. The intensity of microbial activity was controlled by the SO and DOC concentrations in the overlying water. In anaerobic systems, which had SO and DOC concentrations higher than 47 and 7 mg/L, respectively, microbial activity was promoted by SO and DOC; As(III) was desorbed under these indoor simulation conditions. When either the SO or DOC concentration was lower than its respective threshold of 47 or 7 mg/L, or when either of these indices was below its concentration limit, it was difficult for microorganisms to use SO and DOC to enhance their own activities. Therefore, conditions were insufficient for As desorption. The migration of As in lake sediments was dominated by microbial activity, which was co-limited by SO and DOC. The concentrations of SO and DOC in the overlying water are thus important for the prevention and control of As pollution in lakes. We recommend controlling SO and DOC concentrations as a method for controlling As inner-source pollution in lake water.